The sound of merging supermassive black holes does not saturate the universe.

For the past decade, scientists with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration have been listening for a constant “hum” of low-frequency gravitational waves.

Theoretical work suggests that this hum — generated by collisions involving supermassive black holes, which contain millions or billions of times more mass than the sun — should be detectable at Earth. NANOGrav hasn’t heard the hum yet, a new study reveals, but this lack of detection is an interesting result in its own right, revealing new details of how galaxies might evolve and merge, team members said. [The Search for Gravitational Waves in Pictures]